Method of making a semiconductor device having a functional capping

ABSTRACT

A wafer level method of making a micro-electronic and/or micro-mechanic device, having a capping with electrical wafer through connections (vias), comprising the steps of providing a first wafer of a semiconductor material having a first and a second side and a plurality of holes and/or recesses in the first side, and a barrier structure extending over the wafer on the second side, said barrier comprising an inner layer an insulating material, such as oxide, and an outer layer of another material. Then, metal is applied in said holes so as to cover the walls in the holes and the bottom of the holes. The barrier structure is removed and contacts are provided to the wafer through connections on the back-side of the wafer. Bonding structures are provided on either of said first side or the second side of the wafer. The wafer is bonded to another wafer carrying electronic and micro-electronic/mechanic components, such that the first wafer forms a capping structure covering the second wafer. Finally the wafer is singulated to individual devices.

The present invention relates to packaging of semiconductor devices, andin particular to a capping substrate comprising functional features.

BACKGROUND OF THE INVENTION

In so called packaging of semiconductor devices it is sometimes requiredto enclose various components in a controlled atmosphere, i.e. to seal acavity in a controlled atmosphere, in some cases, even most often toform a hermetic seal. This procedure entails bonding two wafers togetheroften under pressure and with heating. This is a delicate task whenwafers are thin because they easily break.

Representative prior art for this field of technology are WO2007/089206, WO 2008/091220, WO 2008/091221, and SE-0900590-1 (notpublished) all assigned to Silex Microsystems, and WO 2009/005462(Nilsson et al). These documents describe various aspects of throughsilicon insulator technology, such as through silicon vias (TSV), “zerocross-talk” and wafer level micro-scale packaging of discrete ormonolithic integrated components.

In US 2004/0259325 (Qing Gan) there is disclosed a wafer level chipscale hermetic seal package. It comprises providing a capping structurehaving a cavity for housing components on a device substrate, and viasextending through the capping structure for routing electrical signalsfrom the cavity and through the capping. The vias appear to be made byetching holes entirely through the wafer and then filling the vias byelectroplating Cu in them. The depth of these vias is said to be limitedto 20-300 μm and the cross-sectional dimension 5-50 μm.

SUMMARY OF THE INVENTION

The present invention provides a method of making semiconductor devicescomprising micro-mechanical and micro-electronic components, includingbut not limited to CMOS components, NMOS, PMOS, bipolar, thin or thickfilm passive or active devices in micro-scale and/or MEMS componentssuch as inertial structures, gyros, accelerometers, switches inmicro-scale, in a hermetically sealed cavity, without risk of damagingthe capping structure. It also provides for integrating various types ofelectric functionalities in the capping structure, functionalities thatare not process compatible with the temperature sensitive CMOSstructures. Also the prior art technologies making use of thin filmstructures for passive integrated devices such as resistance inductorsand capacitors most often require large surface areas and hence are notcost effective.

Thus, in a first aspect the invention provides a general method ofmaking semiconductor devices comprising micro-mechanical and/ormicro-electronic components, having a capping structure. The methodaccording to the invention is defined in claim 1.

In one embodiment the method according to the invention entails using anSOI (Silicon On Insulator) wafer for making the capping structures,whereby the vias are made in the layer defined as the device layer,while maintaining the other layer defined as the handle layer. Thisapproach will ascertain stability and robustness in the process, andsubstantially reduce, if not entirely eliminate the risk of damage ofwafers during manufacture.

In another embodiment of the method according to the invention anordinary silicon wafer is used without the need for carrier handling,but this will require a thicker substrate in order to provide thenecessary robustness for enabling the handling during processing of thewafer.

In a second aspect the invention provides a functional capping structurefor a semiconductor device comprising micro-mechanical and/ormicro-electronic components, such as a MEMS and/or CMOS device, thecapping structure preferably has a hermetically sealed cavity forhousing the components on such devices. This structure is defined inclaim 19.

Also according to the invention there is provided a micro-mechanicaland/or micro-electronic(MEMS and/or CMOS) device having a capping,preferably with a hermetically sealed cavity, defined in claim 21.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1-13 shows a process flow for one embodiment of the methodaccording to the present invention;

FIG. 14-19 shows a process flow for another embodiment of the methodaccording to the present invention

FIG. 20-27 shows a process flow for still another embodiment of themethod according to the present invention;

FIG. 28 shows a capping structure.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is based on the inventive concept of using amethod for making a “metal via substrate”, i.e. a substrate havingimpedance adapted electrical through connections of metal, for RFapplications, and in the same process sequence in said method optionallymaking a plurality of passive components, e.g. resistors, capacitorsand/or inductances, said passive components extending through thesubstrate. Such a metal via substrate is suitable for use in hermeticcapping of electronic and micro-electronic/mechanic structures ingeneral, including but not limited to CMOS components, NMOS, PMOS,bipolar, thin or thick film passive or active devices in micro-scaleand/or MEMS components such as inertial structures, gyros,accelerometers, switches in micro-scale, CMOS and/or MEMS devices, e.g.CMOS structures comprising switches.

The invention can be used to make a single transmitter/receiver chipintegrated with switches and RCL filters that filters out correctfrequency and switches it on to the antenna in the mobile phone or intothe receiving chip. Provision of such RF switching enables selection ofwhich band (frequency) one wishes to use; 900 MHz (GSM) 1800 or 1900 MHz(3G Europe and US, respectively), 2800 MHz and other frequencies forBluetooth and the different WLAN standards. The RLC devices could thenalso be used as antennas or integrated antennas for sending andreceiving signals.

Generally, the method according to the present invention for makingsemiconductor devices comprising micro-mechanical and micro-electroniccomponents an having a capping structure for such devices, e.g. CMOSand/or MEMS devices, comprises the following steps.

A starting substrate in the form of a semiconductor wafer, preferably asilicon wafer is provided, having a first and a second side. The waferhas been processed to a plurality of holes and/or recesses in the firstside, and a barrier structure extending over the essentially entirewafer on the second side, said barrier comprising an inner insulatinglayer, suitably oxide but other insulating materials capable ofwithstanding the processing are usable, and an outer layer of anothermaterial. The alternative processing methods for arriving at thestarting substrate will be described in more detail below, in connectionwith embodiments of the invention.

Thus, in one embodiment the holes in the starting substrate can have auniform cross-section, such as a cylindrical hole, and in anotherembodiment the holes can have a section exhibiting one diameter and asecond section having a smaller diameter.

Furthermore, the starting substrate has a barrier structure on thesecond side, said barrier structure forming the bottom of the holes.This barrier structure can extend essentially over the entire wafer onthe second side, but a minimum requirement is that it covers the holes.The barrier comprises an inner layer of an insulating material, such asoxide, and an outer layer of another material.

Metal is applied in the holes, only covering the walls and the bottom ofthe holes or trenches, i.e. not filling the holes entirely. Also thebottom of the holes will be provided with metal thereby obtaininghermetic type connections. The reason for this being that if the holedensity is large (sometimes as many as several hundred thousand holescan be provided on one wafer), entirely filled holes would cause severeproblems during processing. Namely, process temperatures would causeheating of the wafer, and would cause expansion of the metal in theholes to a degree that the silicon wafer would crack. However, in theembodiment having holes with a narrow section the narrow holes arepreferably entirely filled with metal. Since these sections are narrow,the filling will not have the same effect upon heating as if the widerholes were filled. The holes now coated inside with metal formstructures that eventually will be wafer through connections (vias).

The barrier is removed after the metal has been applied and the waferdoes not have any open through holes. Open through holes prevent the useof ordinary IC manufacturing equipment such as cassette-to-cassettewafer handling with robots and electrostatic or vacuum clamping of thewafers on different chucks in the processing machines.

Larger recesses can be made in the wafer, at least some of which areadapted to form cavities with a controlled atmosphere, preferablyhermetically sealed, in the finished device to confine micromechanicaland/or microelectronic components of e.g. a CMOS (in a general sense)device and/or a MEMS device.

However, if the components on the wafer to which the capping is to bebonded do not protrude very much or perhaps not at all, it may besufficient to use the bonding structures as spacers to create a spacelarge enough for accommodating the components. Thereby the height of thebonding structures can be adapted to the wafer topography.

The wafer having metallised holes and/or trenches is bonded to a waferhaving components provide on its surface, e.g. CMOS and/or MEMS waferwhereby at least some of said recesses will house the active componentson the CMOS and/or MEMS wafer.

If desired, routing structures are provided on the surface(s) of thewafers, preferably before they are bonded together, and optionally padbumping suitable for surface mounting.

Now a preferred process for making a device having a capping substrateas disclosed above and comprising optional functionalities will bedescribed. The process utilizes a so called SOI (Silicon On Insulator)wafer as a starting substrate, FIG. 20. An SOI wafer has a relativelythin so called device layer DL in which processing is performed, and amuch thicker handle layer HL, for ease of handling the wafer. Betweendevice layer and handle layer there is an insulating, buried oxide layerBOX, which acts as a etch stop layer in the processing. The handle layerand the buried oxide together form what above is referred to as thebarrier structure, and the handle layer as such also has the function ofa carrier. The handle layer is subsequently removed. Layers are notshown to scale.

A first step in the process according to this embodiment of theinvention is to pattern the device layer of the SOI wafer as requiredfor making the components. For example parallel trenches are formed(suitably by DRIE; Deep Reactive Ion Etching) for the purpose of makingcapacitors and for making the cores for inductances, and holes areetched for providing via structures. The trenches and holes 22 areetched down to the insulating etch stop layer BOX. In this way welldefined trenches and holes are obtained.

Then the entire wafer is oxidized to provide a thin insulating layer 24on the wafer and in all holes and trenches. A seed-layer 26 ofconductive material, such as metal, e.g. Cu or Au provided e.g. bysputtering, evaporation or plating, or plasma deposited poly-silicon,for facilitating subsequent metallization, e.g. by electroplating orelectroless plating. This seed-layer also can act as an interdiffusionmetal barrier avoiding the thick metal in the through wafer connectionto diffuse laterally into the insulator and further into the silicon.

In order to define where the metallization is to be deposited, a resistlayer 28 is applied to the wafer, either by film lamination of a resistfilm or by spin coating.

Areas 30 for metallization are opened up in the resist 28, and metal isdeposited by e.g. electroplating. As metal Cu is suitably used, but Auis also possible. It is important that the metal is coated as a layer onthe inner surfaces of the holes only, such that the vias are not filled,i.e. they will not be solid but still hermetically tight.

If Au is used to make the metal via structures, subsequent bonding witha component wafer, e.g. a CMOS/ MEMS wafer, can be made directly to thedeposited Au. However, if Cu is used there is required to provide asolder structure.

Thus, when the Cu metal has been properly deposited in the vias, theresist mask 28 is removed and a new resist layer 34 is applied, again bylamination or by spinning. Holes and recesses 36 are opened up in theresist, thus forming what is referred to as a resist mold, and bondingmaterial 38 is applied, suitably by plating in the resist mold. Theresist 34 is removed and the capping structure shown in FIG. 26 results,having suitable bonding members 40 extending up from the surface.

In order for the capping structure to be able to accommodate anycomponents 42 on the component wafer that protrudes above the wafersurface, one can create cavities in the capping structure. In FIG. 27such a cavity is indicated with a broken line. However, if thebonding/soldering structures are sufficiently high, as indicated in FIG.27, the space provided between the wafers may suffice for accomodatingthe components without having to create cavities.

After having bonded the wafers together to arrive at the structure shownin FIG. 27, the handle is removed, but the oxide is left.

However, in order to provide contact to the wafer through connections,the oxide is opened up by suitable masking and etching through the oxidedown to the metal. Again the wafer is patterned to provide a maskdefining contact pads and other metal structures such as routing. Metalis applied in the holes and on the wafer whereby the desired metalstructures are formed.

Again the surface is patterned with resist and suitably etched toprepare for applying a passivation layer on top of the wafer so as tocover the metal structures. Finally bumping is provided at desiredlocations by opening up the passivation layer to expose metal forattaching solder bumps. The final result is shown in FIG. 27 b.

Alternatively, the contacts can be made by etching through the oxidewhen the holes are made. This requires two etch steps, one for thesilicon and one for the oxide. Thereby the oxide etch is applied suchthe a slight “over-etch” into the handle is made.

Then the process is as for the ordinary process.

When the handle is removed, the metal in the through connection willprotrude slightly above the buried oxide layer, and e.g. by grinding aflat surface having contact points will be provided.

The final result will be very similar to the result shown in FIG. 27 b.

Suitably the next step is to make the core for the inductance if such isdesirable.

To this end the entire wafer is masked and patterned to expose only thetrenches for making the core. The masking can be made by covering theentire wafer with a film or by spinning a resist onto the wafer. Themask is opened up over those trenches that are to form the inductancecore. Suitably electroplating is employed for filling the trenches withthe desired metal. Preferably a Ni/Co alloy is used for this purpose.Thus, the core extends vertically into the wafer.

The inductance functionality described above can also be provided inother ways. For example such structures could also comprise a thininsulating segment, i.e. a filled trench, extending through thesubstrate but running in a helical pattern so as to form an inductancecoil.

Next the wafer is masked and patterned as described above to exposeremaining structures, i.e trenches for capacitor plates and for viastructures. Again, plating is used for growing Au, Cu or Al to athickness of at least a few μm with low resistivity. This will in mostcases leave a void inside the holes/trenches. Again, the structurescreated extend vertically into the wafer meaning low area consumptionand thereby cost efficiency.

Preferably but optionally these voids are filled with a material that iscompatible with the substrate wafer material in terms of coefficient ofexpansion, such that thermal impact will not cause the substrate tocrack.

Suitable materials for filling are oxide (e.g. TEOS) or poly-silicon.Also polymers such as BCB, SO8 or polyimide are usable.

In a particularly preferred embodiment of the invention, routingstructures, i.e. structural elements for connecting components, are madeon the substrate in the same process sequence as described above. Suchrouting structures are narrow strips of metal.

There are two alternative procedures for making these routings.

In one embodiment the initial step of patterning and etching thetrenches and holes is divided in two substeps. This is disclosed in WO2007/089206. First the wafer is patterned to define the routingstructures, and the wafer is subjected to an etch to a depth of only afew μm. Thereby shallow recesses or grooves are provided in the surface.Then the wafer is patterned again to provide the trenches and holes, asdescribed above.

The reason for using this sequence is that it would be more difficult tospin resist on the more complicated topography provided by the deeptrenches and holes, although the latter is possible. Also bonding of thecapping wafer is more difficult with topography present.

When the metal is deposited onto the substrate the recessed will beentirely filled with metal and form conductive strips. In this way therouting structures will be provided in a recessed manner, i.e. they willbe provided in the substrate surface rather than on it.

In an alternative embodiment the routing will be provided after theother structures have been made. Hereby the entire wafer is patternedafter the final step of filling the voids (if applied), to define therouting structures. Metal is deposited on the wafer in the openings inthe pattern. In this alternative the routing structures will be provideon the surface of the substrate.

These methods of making routing structures are suitable also forproviding the metal strips forming an integral part of the winding (onthe device layer side of the substrate) for the inductance as describedabove.

The metal strips for the inductance on the opposite side of thesubstrate are made after the capping substrate has been bonded to theCMOS/MEMS wafer, and will be described below.

An alternative method of making a capping structure as discussed above,is to refrain from using a SOI wafer as a starting substrate and insteadstart from a plain wafer. In this method the wafer thickness is at least350 μm, suitably the thickness ranges between 200 and 1000 μm in orderthat the wafer can be handled in the process without risk of breaking orthe need of special carrying systems.

Such a process will now be described with reference to the drawingsFIGS. 1-13.

The first part of the process is to make vias (wafer throughconnections) in the starting substrate 10. This is schematicallyillustrated in FIG. 1. One side (here referred to as the “front side”FS) of a wafer is covered with a resist mask 12. This can either be inthe form of a resist layer that is spun onto the wafer, or it can be alaminated resist film. A suitable thickness of the resist is 10 μm.

The resist film/layer is suitably patterned and exposed to open up holes14 in the resist so as to define the via holes. The diameter of theholes can vary from 20 μm up to 150 μm. An etch, suitably Deep ReactiveIon Etch (DRIE) is then applied to the wafer through the resist mask toform holes with a depth in the range 270-570±5 μm.

It is important to have control over the etch parameters in order thatthe via will exhibit the desired properties. One of the most importantproperties to control is the wall slope in the hole that is beingetched. The wall should preferably be 90°, i.e. the wall should extendvertically into the wafer. It is acceptable that it narrows slightly,i.e. that the wall has a “positive” slope. However, a “negative” slopeindicating that the hole widens inwardly is not acceptable. The reasonis that subsequent steps (e.g. plating) in the process will not functionproperly if the slope is “negative”.

Also the wall roughness is an important property to control, in order tobe able to achieve a continuous seed layer.

When the holes have been properly etched the resist is stripped away byconventional means (dissolving, etching etc.), and the wafer issubjected to thermal oxidation or similar processes for applying films,schematically illustrated in FIG. 2. Thereby a thin oxide layer 20 isgrown on the wafer on the side of the wafer having the vias only. Theoxide covers the walls and the bottom of the holes.

At this stage the wafer comprises holes extending into the wafer but notall the way through. In order to provide wafer through vias to enablethat electrical signals are transferred through the wafer, these holesmust be internally provided with metal, either as a coating on a wall orpossibly by filling the entirely, and also providing a furtherconnection to the other side.

Filling the holes entirely with metal will in cases of close packed viasbring about severe tension in the wafer due to the different thermalexpansion coefficients of metal and silicon, respectively. Thus, it isdesirable to make use of the wall coating alternative and still havingclosed holes.

For providing the required connection to the back side, the followingprocess is performed.

As shown in FIG. 3 in the next step the wafer 10 is masked with resist30, suitably 2 μm thick, on the other (back) side BS of the wafer todefine openings 32 located such that they are essentially concentricwith the vias. Suitable alignment is of course required, but suchmethods pertain to the field of the skilled man, and are thus not partof the present invention per se, and will not be discussed herein.

These openings form a pattern defining “back side” via holes. Etchingthrough the openings in the mask will then provide holes which aresubstantially smaller than the via holes on the opposite side, typicallybetween 8 and 16 μm in diameter.

As can be clearly seen in FIG. 3, the thermal oxide 20 in the “frontside” via holes 14 will act as an etch stop layer for the narrow “backside” holes 32.

Etching of these “backside holes” is suitably made by DRIE and can beperformed in different ways, essentially one of two: with or without“trumpet etch”. A “trumpet etch” entails rounding of the circumferentialedges of the holes in order to facilitate subsequent filling by plating.

The resist is stripped away by etching or dissolving and the oxide layeris also removed, suitably by HF etch, and the result is a “naked wafer”10, shown in FIG. 4, with a through hole comprising two sections, onedeep and wide section 14 on the front side and one shallow and narrowsection 32 on the back side of the wafer. Again, the entire wafer isoxidized to provide a layer about 5000Å thick (not explicitly shown inFIG. 5-13)), onto which a seed layer is deposited, suitably but notlimited to a sequence of layers comprising:Ti100Å/TiN400Å/Ti100Å+Cu4000Å, schematically illustrated in FIG. 5.Preferably the seed layer is deposited by sputtering from both sides ofthe wafer.

Now, with reference to FIG. 6, the via holes will be coated internallyon the walls filled, at least partially, with Cu by a plating process.Of course other metals are usable, e.g. gold. This is a single sidedprocess, i.e. the plating is allowed to act only on one side, namely thefront side FS where the deeper and wider holes are provided. To enablesuch one sided plating the front side is covered with a film 60 ofresist which is suitably laminated (glued) to the wafer, which is shownin FIG. 6. Spinning the resist onto the wafer is not possible since theresist material would then enter into the via holes and it would berather difficult to remove the resist from the holes before the etching.Also the back side is covered with a resist film 62 in the same way. Theback side resist together with the oxide on the back side form thebarrier structure mentioned earlier.

The front side resist is then patterned to open up areas 64 for plating.

Here not only the via holes are exposed, but the pattern also compriseslines and other structures for routing between vias but also betweenvias and more remote areas on the wafer if needed for the purpose ofenabling attachment of other components.

The plating process is preferably a so called fountain plating process(SEMITOOL PARAGON™ plating systems and alike), which comprises flushingthe wafer front side with a plating solution while applying a voltage tothe wafer. In this way the plating solution efficiently penetrates intothe via holes which have a fairly high aspect ratio, which would bedifficult using a conventional immersion based process, which createsproblems due to surface tension effects.

As can be seen in FIG. 6, the plating metal (e.g. Cu or Au) 66 extendsinto the front side via hole and covers the walls, but it alsopenetrates further, partially up into the back side via holes. In mostcases the plating metal will fill the entire diameter of the narrow backside hole to provide a hermetic seal, but it cannot be guaranteed to100% that this is the case. Therefore, a second plating form the backside is also performed, as illustrated in FIG. 7.

Here, the front side is entirely covered with a laminated resist film70, so as to enable a single sided plating on the back side. For thispurpose the back side is provided with a laminated resist film 72, andthe resist is opened up 74 at the back side via holes as shown. However,it is to be noted that the opened up area is slightly larger that theactual via hole. Then a plating process is applied whereby the back sideholes are filled with metal 76, suitably Cu or Au, and also a portionaround the holes are plated to provide a “collar” forming a suitablecontact pad for attaching further components. Of course the resist 72can be patterned so as to also provide routing structures on the frontside if desired.

The resist layers on both sides are now stripped off (see FIG. 8).

Since the resulting device is a capping structure, it requires bondingstructures. Typically bonding of the capping structure to anothersubstrate is achieved by thermo compression, or soldering, bond padsmust be provided.

In particular it is required that there be provided sealing structuresso as to enable the capping to provide a hermetic sealing of componentsinside the capping.

Thus, the wafer is again covered with a resist film 90 on the front sideby lamination, and holes and areas are opened up to provide a maskdefining the bond pads/bonding/sealing structures, as shown in FIG. 9.Then, stacked metal layer structures 100 are applied through the mask byelectroplating, as shown in FIG. 10. Suitably the structure is made upof a first layer of Ni, a second layer of Au and a final layer of Sn.This process is a single sided process. The mating hole makes the waferhermetic and solid.

The resist is removed, see FIG. 11, and again a resist film 120 islaminated onto the wafer front side, see FIG. 12. This resist is alsopatterned thus forming a mask which define cavities 122 in the wafer,said cavities being provided so as to house components on the surface ofthe wafer to which the capping substrate is to be attached. A DeepReactive Ion Etch (DRIE) is applied through the openings in the maskwhereby the cavity forms. FIG. 13a shows the final result of theprocess, i.e. a capping structure 130 having vias 132 for routingsignals from devices provided inside a hermetically sealed cavity.

FIG. 13b is a SEM image of a capped device according to the inventionfabricated with 200 mm diameter silicon wafers. The bonding ispreferably made on an EVG Gemini bonder alike.

By the method disclosed above a hermetic seal of the wafer through viais ascertained, as with the previously described method when an SOIwafer was used as a starting substrate.

It should further be noted that a via structure having hermetic sealingproperties can be made by other methods. One example is disclosed ininternational patent application WO 2009/005462 (Assignee: ÅSTCAerospace AB). The method claimed in this application enables using anordinary plain wafer as a starting substrate, and to position thehermetic seal at optional position vertically in the wafer through via.

In a variation of the above described method, it is possible to make ahermetic seal around each via structure using very thin “knifestructures” encircling the vias. This variation is shown in FIGS. 14-19.The general methods above are used to make the vias, but the wafercontaining the vias is turned “upside-down” compared to the abovemethod, illustrated in FIG. 14. However, the very thin “knife”structures are made on the back-side of the wafer having the vias, i.e.on the side where the narrow via portions are provided.

In order to make such knife structures it is not possible to laminateresist onto the wafer, since the laminated film is too thick, and it isnot possible to make the very thin structures forming the knives in alaminated resist. Thus a spinning method is preferably used to applyresist. Spinning requires an essentially flat surface, and thus thefront-side of the wafer cannot be used for this purpose.

However, before the knives are made, a seal ring should be made forbonding the wafers together.

As shown in FIG. 15, resist 150 is spun onto the back-side BS of thewafer. The spun resist is patterned and grooves/trenches 152 are openedup in the resist. Said grooves are subsequently filled with suitablematerial, such as solder comprising Au/Sn, to form the required sealring 154.

The resist is removed (FIG. 16) and a new resist 160 is spun onto thewafer (FIG. 17a ), patterned to define the knife structures, grooves areopened up, and metal 162, preferably gold (Au) is deposited in thegrooves, to provide the knives.

Suitably the seed layer that is still on the surface is removed byetching, either maskless or with a protecting resist mask. In FIG. 17b-cthe resist mask from FIG. 17a is removed (FIG. 17b ), and a new resist163 is applied to protect the knife structure and then the seed layer isremoved (FIG. 17c ).

Then, the recesses forming the cavities for the housing the componentson the other wafer are made after the knife structures have been made(see FIG. 18). Also here resist 180 is spun onto the wafer back-side,patterned to define the cavities 182 and etch is applied through theopenings in the resist to make said cavity recesses.

FIG. 19a shows a finished capping structure, and FIG. 19b shows a bondedstructure comprising the capping from FIG. 19a bonded to a wafer havinga MEMS structure MS and a knife structure KS and a sealing bond SB.Also, the fact the narrower part N of the through connection is filledwith metal, which provides a sealing itself, in combination with theknife structure KS surrounding the through connection, creates aredundancy in the sealing, such that if one of the seal is defect, theother will still provide sealing. As indicated there can be concentricknife structures KS 1, KS2 to further increase redundancy.

FIG. 28 is a cross-section in a perspective view of a functional cappingsubstrate according to an embodiment of the invention, i.e. beforehaving been bonded to a CMOS and/or MEMS device wafer. It should benoted that in FIG. 28 the upper oxide layer is not shown for clarity.Thus, all features shown as slightly protruding from the surface areburied in the oxide, and only the upper surfaces thereof are exposed.E.g. the inductance windings appear to be “free hanging” but are in factsupported on the oxide layer.

Thus, the shown device comprises generally a capping substrate 281, i.e.a cover structure for encapsulating, suitably hermetically sealing, CMOSor MEMS structures 282 (indicated below the capping substrate).

The capping substrate 281, suitably made of high resistivity silicon,although other materials are possible, comprises several functionalcomponents. There are also provided relatively wide recesses R forproviding the compartments in which the CMOS/MEMS components are to beoperated in a controlled atmosphere when the capping structure is bondedto a component wafer.

The primary functional feature is the provision of the wafer throughconnections, or via structures, generally designated 283.

In the most general form the metal via is a simple via, i.e just a metal“plug” extending through the substrate.

However, a “plug” is not necessarily solid but can be formed by coatingthe walls only of a hole.

For RF applications these vias are suitably made as coaxial electricalconnections extending through the substrate thereby obtaining impedancematching.

In a further embodiment such coaxial vias comprise a metal “plug”extending through a wafer, with a thin insulting layer of e.g. oxideprovided between metal and wafer material. At a radial distance there isprovided an annular metal structure, thus enclosing the central metal“plug”. This annular metal structure is also preferably insulatedagainst the wafer material by thin insulating layers at both the innerand outer circumferences. The annular metal structure forms a shield,and together these structures form a coaxial through connection,providing impedance matched properties for RF signals.

In some embodiments of the coaxial connections (shown in FIG. 28) thecentral “plug” itself comprise a central portion 284 of a material thatis compatible with the material in the wafer from which the substrate ismade, e.g oxide (TEOS) or poly-silicon (or any other material that willhave a similar coefficient of expansion as the wafer material from whichthe substrate is made). This central portion is surrounded by the metal284′ in an annular structure, through which electrical signals can betransferred. Thus, in this case the “plug” is a composite structure.

For very small dimensions the central plug can be entirely made frommetal, i.e. there is no void created during manufacture that Isubsequently filled, but the metal will fill the via entirely duringmanufacture.

Between metal and substrate material in the respective structures thereis a thin oxide layer (not shown), so as to electrically insulate fromthe silicon in the substrate 281.

Furthermore, there is a structure 285, 285′ arranged concentricallyaround and at a radial distance from the insulated metal via 284.Between the via 284 and this structure 285, 285′ there is an annularsilicon portion 284″ surrounding the metal via 284′. The concentricstructure is indicate at 285″.

The concentric structure 285, 285′ comprises two concentric, annularmetal structures 285′ between which (i.e. at 285) there is provided thesame material as in the central portion 284, i.e. oxide (e.g. TEOS) orpoly-silicon (or any other material that will have a similar coefficientof expansion as the wafer material from which the substrate is made).

The annular metal structure 285′ will function as a shield to thecentrally located via 284. Properly designed the impedance in suchstructure becomes 50 Ohm allowing RF signals transmitted in the metalvia with minimum reflection and damping.

The overall structure will be that of a coaxial connection between thetwo sides of the capping substrate 281, thereby forming an ohmicconnection between the MEMS/CMOS devices through the capping substrateto external devices.

Another functional feature of the capping substrate can be the provisionof a capacitor structure 286 within the substrate. Such a capacitorstructure is provided by having metal provided in thin segments 287 (sixshowing in FIG. 28) extending preferably all the way through thesubstrate and also extending across the plane of the substrate. Ifseveral such segments are arranged next to each other and in parallel asshown in FIG. 1, with only a very small spacing between them, thematerial segments 288 of the substrate between these insulating segmentswill have the function of capacitor plates.

A further functional feature, shown generally at 289, is the provisionof a “spiral” structure forming a coil to provide an inductance.

The inductance comprises a metal core 2810 made of e.g. Ni or preferablya Ni/Co alloy, said core preferably extending through the substratethickness, and having an elongated shape, extending essentially in theplane of the substrate. Furthermore, there is provided a winding aroundthe core consisting of a combination of a plurality of via structures2811, arranged in arrays along the metal core and extending through thesubstrate, and provided on both sides of the core 2810, and metal strips2812 connecting via structures 2811 pairwise across the core 2810. Byletting the first via on one side of the core and on the upper surfaceof the substrate (as seen in the figure) connect to an opposing viaprovided on the other side of the core and then connecting said opposingvia on the bottom side of the substrate (as seen in the figure) with anadjacent via to the first mentioned via, and so on, i.e. providing anessentially “zig-zag” connection between vias, a “spiral” woundconductor is provided around the metal core, thereby effectivelycreating an inductance.

In FIG. 28 there is shown a double array of vias arranged in a staggeredconfiguration. This enables a closer spacing of metal strips, since thestrips can be made narrower than the diameter of the vias themselves. Itis possible to provide triple or even quadruple arrays of vias. In thisway the number of turns of the winding can be increased substantiallyand the properties of the inductance can be taylored to a higher degree.

The monolithic integrated capacitor and inductance features forreplacing discretely mounted components. Using high effectivecapacitors, inductors and resistors various decoupling or filteringfunctions can be integrated.

For enabling a thermo compression bonding of the capping substrate toe.g. a CMOS/MEMS device it may be required to provide a metallization2820 that runs circumferentially around the area defining the finaldevice, corresponding to a mating metallization on the CMOS/MEMS device.Such metallization is preferably Au or Cu.

Also various eutectic bonding approaches are possible, e.g.Au/poly-silicon, AuSn/Au, or many other eutectic alloys (see patentapplication SE-0900590-1, not published).

A further preferred feature according to the invention is to actuallynot stop etching at the etch stop layer of the SOI wafer when holes andtrenches are made in the initial stage of the process, but in fact tocontinue etching further down into the stop layer. It is also possibleto etch through the stop layer entirely.

The advantage of this is that when the handle layer finally is removed,the metal deposited in the via holes will become exposed and can formcontact surfaces without the need for further processing, such aspatterning and etching to expose the metal. The exposed metal can thendirectly be plated or processed in other ways to provide pads 2822 forelectrically coupling the CMOS/MEMS components to the via.

When all the desired functions and components have been made in thedevice layer of the SOI wafer, the entire wafer is optionally patternedagain to define the wider recesses R that are to provide thehermetically sealed compartments with a controlled atmosphere, if suchcavities are required by the topography of the component wafer. Suitableetching to a desired depth will result in appropriate hermeticallysealed compartments.

At this stage the capping substrate is still provided with the SOIhandle layer. Now it is to be bonded against the CMOS/MEMS substrate282. To this end the circumferential metallizations are matched andpressed against each other whereby a hermetic tight seal 2820 is formed,either by thermo compression or by eutectic melting bonding.

After bonding together the capping structure and the CMOS/MEMS device,the handle layer is removed in conventional way, e.g. grinding oretching or any other method well known to the skilled man.

As already indicated, if the hole and trench etching was made throughthe insulator layer of the SOI wafer, the metal deposited in the holesand trenches will be exposed and form suitable contact points forproviding contact pads for further connection. At this stage also backside routing is made in a similar manner as already described above.

It should be noted that the cross connecting metal strips of theinductance is now made, suitably at the same time as the other routingstructures. By making bumps 2828 of solderable material (e.g. Ni/Au),surface mounting for example flip-chip type mounting will becomepossible.

In a further preferred embodiment there can be provided insulatingenclosures surrounding selected elements in the capping substrate. Suchinsulating enclosures prevent “cross-talk” between components andregions, thereby minimizing signal strength losses. Methods for makingsuch enclosures are disclosed in applicants own international patentapplication WO 2008/091220.

The invention claimed is:
 1. A wafer level method of making amicro-electronic and/or micro-mechanical device, having a cappingstructure with electrical wafer through via connections, comprising thesteps of: providing a first wafer of a semiconductor material having afirst side and a second side and a plurality of holes and/or recesses inthe first side, and a barrier structure extending over the first waferon the second side, said barrier structure comprising an inner layer ofan insulating material and an outer layer of another material; applyingmetal in said holes so as to cover the walls in the holes and the bottomof the holes; removing the barrier structure and providing contacts tothe first wafer through connections on the second side of the firstwafer; providing bonding structures on either of said first side or thesecond side of the first wafer; bonding the first wafer to a secondwafer carrying electronic and microelectronic/mechanic components, suchthat the first wafer forms a capping structure covering the secondwafer; and singulating the bonded first and second wafers to individualdevices; wherein the first wafer is a semiconductor wafer, and whereinthe barrier structure comprises an oxide layer on the second side of thefirst wafer, and a resist layer covering the first wafer.
 2. The methodas claimed in claim 1, wherein the resist layer is a dry resist.
 3. Themethod as claimed in claim 2, wherein the resist layer is applied bylamination of a film.
 4. The method as claimed in claim 1, comprising:providing the first wafer having a first and a second side; providingholes and/or trenches in the first wafer; providing a thin insulatinglayer inside the holes covering the walls and the bottom of the holes;providing wafer through structures by: making mating holes on saidsecond side so as to provide through connections applying an insulatingmaterial on the first wafer; providing a seed layer; applying metal inthe holes/trenches such that the metal only covers the surfaces of thewalls and bottom, in the holes or trenches to complete the wafer throughstructures, whereby a capping structure is finished; and bonding thefirst wafer having metallised holes and/or trenches to an electronic andmicroelectronic/mechanic wafer.
 5. The method as claimed in claim 4,comprising: patterning the first side using resist and standardlithographic techniques, or other suitable techniques, to create a maskfor defining areas where wafer through connections in the form of holesand/or trenches of various shapes eventually are to be made; etchingholes and/or trenches in the first wafer through the openings in themask; removing the mask; and providing a thin insulating layer.
 6. Themethod as claimed in claim 4, further comprising providing a sealingstructure around the first wafer through connections and the bondingstructures on said second side where the mating holes are provided. 7.The method as claimed in claim 6, wherein the sealing structure incombination with the metal in the mating holes provide redundancy insealing.
 8. The method as claimed in claim 6, wherein the sealingstructure is provided by a combination of metal in said mating holes andat least one sealing structure surrounding the through connections. 9.The method as claimed in claim 1, wherein the metal application in theholes and/or trenches is by electroplating or electroless plating. 10.The method as claimed in claim 9, wherein the metal is selected from Cuand Au.
 11. The method as claimed in claim 1, wherein the semiconductorwafer is a silicon wafer.
 12. A wafer level method of making amicro-electronic and/or micro-mechanical device, having a cappingstructure with electrical wafer through via connections, comprising thesteps of: providing a first wafer of a semiconductor material having afirst side and a second side and a plurality of holes and/or recesses inthe first side, and a barrier structure extending over the first waferon the second side, said barrier structure comprising an inner layer ofan insulating material and an outer layer of another material; applyingmetal in said holes so as to cover the walls in the holes and the bottomof the holes; removing the barrier structure and providing contacts tothe first wafer through connections on the second side of the firstwafer; providing bonding structures on either of said first side or thesecond side of the first wafer; bonding the first wafer to a secondwafer carrying electronic and microelectronic/mechanic components, suchthat the first wafer forms a capping structure covering the secondwafer; and singulating the bonded first and second wafers to individualdevices; wherein the first wafer is an SOI wafer, and wherein thebarrier structure comprises a handle layer and a buried oxide layer ofsaid SOI wafer; and further comprising: patterning and etching a devicelayer of said SOI wafer down to an insulating stop layer / the buriedoxide layer to provide a structure comprising a plurality of holesand/or trenches having a well defined depth; applying metal in the holesand/or trenches; providing the second wafer having electronic andmicro-electronic/mechanic device structures provided thereon; bondingthe first wafer and the second wafer together; and removing a handlelayer of the SOI wafer.
 13. The method as claimed in claim 12, furthercomprising making recesses in the device layer before bonding, at leastsome of the recesses being adapted to form cavities with a controlledatmosphere.
 14. The method as claimed in claim 13, wherein the cavitieswith the controlled atmosphere are sealed hermetically by the bondingcomprising provision of matching metallizations on both wafers,surrounding the device in its entirety and which together form a tightjoint when bonded together.
 15. The method as claimed in claim 14,further comprising providing a sealing structure around the waferthrough connections and bonding structures on said second side.
 16. Themethod as claimed in claim 14, wherein said matching metallizations incombination with the metal in the through holes provide redundancy. 17.The method as claimed in claim 12, further comprising after removing thehandle layer, opening the buried oxide layer at the first wafer throughconnections so as to expose the metal therein for forming points ofcontact.
 18. The method as in claim 12, further comprising providingmetal structures, and/or routing structures on the surfaces of the firstand second wafers.
 19. The method as claimed in claim 18, wherein themetal structures form surfaces for attachment of solder bumps.
 20. Themethod as claimed in claim 18, wherein the metal structures are made ona surface of the insulating stop layer after removal of the handle layerand are made by: patterning the surface of the insulating stop layer todefine the structures, and then either: i) applying metal in theopenings in the pattern, or ii) etching down into the surface of theinsulating stop layer to make recesses and then fill the recesses withmetal.
 21. The method as claimed in claim 20, wherein step ii) comprisesetching to a depth of a few μm.
 22. The method as claimed in claim 12,further comprising providing contact pads on the surface of theinsulating stop layer, suitable for surface mounting.